The present invention relates to a lead frame or an electronic device (including also a semiconductor device) which employs the lead frame, and is directed, for example, to a semiconductor device which has a plurality of leads and which is molded in a resin under the state that these leads and a semiconductor chip having a plurality of electrodes are connected by wires.
In a resin-molded type semiconductor device such as IC or LSI, a lead frame is used at the stage of assemblage.
FIG. 2 shows a previously proposed lead frame which is now in use. As shown in the figure, the lead frame is integrally formed with a tab 2 which serves to mount a semiconductor element (chip) 1 thereon, a plurality of leads 3a and 3b (including tab suspension leads 4) which are arranged substantially radiately around the tab 2, a dam 5 which couples the leads, and an outer frame 6 which supports the whole arrangement on the outermost side (such structural arrangement being collectively called the "lead frame"). At a molding step, a structure, in which the desired electrodes of the chip disposed on the tab and the desired leads connected by wires 7 made of, e.g., gold (Au), is molded in a resin. Thereafter, the the dam parts 5 and the outer frame are removed to separate the individual leads, whereby an electronic device is completed.
In this lead frame, the lead portions inside the dam 5 to be molded in the resin are termed the "inner leads" 3a, and the portions outside the dam 5 are termed the "outer leads" 3b.
As partly shown on an enlarged scale in FIG. 3, the inner leads 3a have their fore end parts 3c formed so as to project toward bonding pads 8 which are provided on the peripheral edge of the chip. However, the trend of the packages of semiconductor devices is to have the packages be rendered smaller in size and increased in the number of pins as described in a jounal "Nikkei Electronics" published by Nikkei McGraw-Hill Inc., dated Jun. 4, 1984, pp. 141-152. The inventor's study has revealed that the reduction in size and the increase in the number of pins of the packages cause the following problems with respect to the lead frame of FIGS. 2 and 3:
(1) The spacing A between the dam 5 and the tab 2 as indicated in FIG. 3 shortens, so that a length l in the inner lead 3a from the dam to the connection point of the bonding wire decreases. This results in a shorter invasion path of water, which degrades a resistance to moisture.
(2) The depth B.sub.1 of the inner lead 3a decreases, so that the mechanical and thermal durabilities of the package decrease and that also the resistance to moisture degrades. More specifically, when the depth B.sub.1 decreases, the amount of the resin to be packed in the corresponding space lessens. When a temperature cycle test is conducted under this condition, openings are liable to appear at the interface between the lead frame and the resin on account of the difference of the coefficients of thermal expansion thereof, and eventually, cracks sometimes arise.
Moreover, the resin itself has a water absorptivity. Therefore, when the depth B.sub.1 is smaller, water is more prone to reach the bonding wire to that extent, so the resistance to moisture deteriorates.
(3) As the spacing A between the dam 5 and the tab 2 becomes shorter, the magnitude of projection of the fore end of the inner lead is inevitably made smaller. Therefore, a wire bonding area S decreases to that extent, thereby diminishing a bonding deviation margin.